The present invention relates to a device for driving a print head of an image forming apparatus. More particularly, the present invention is directed to a circuit for generating an adjustable control voltage applied to electrodes in a print head of a charge deposition printing system.
In systems for electron beam imaging and charge deposition printing, a print head having several closely spaced RF electrodes with a number of overlapping, transverse control electrodes (fingers) is commonly used to deposit charges on an imaging member. The print head may be configured to deposit either positive or negative charge, and the negative charge may consist partly or entirely of either ions or electrons. Print heads of this type are described in several U.S. Patents including, for example, U.S. Pat. Nos. 4,160,257; 4,992,807; 5,278,588; 5,159,358 and 5,315,324.
Generally in systems using this type of print head, the RF electrodes are selectively activated with a high-voltage RF drive signal which generates a localized plasma (that is, a localized charge source). The fingers, when maintained at a first potential, retain charge carriers within the charge source. Applying a control voltage to a finger electrode allows the charge carriers to escape from the charge source region at the crossing of the activated RF electrode and the finger. The charges gated from the charge source region are deposited on an imaging member, thereby forming a latent image that may be used to retain toner for transfer to a permanent recording media such as paper. By controlling the application of the high voltage RF drive signals along with the potential of the control voltage applied to the fingers, a specific pattern of charges can be deposited.
The accuracy with which the pattern of charges is deposited upon the imaging member depends, in part, upon the accuracy of the timing, duration and potential of the control voltage applied to the fingers and the accuracy of the RF signals energizing the RF electrodes. Assuming accurate application of drive signals to the RF electrodes, applying a control voltage to the individual fingers for a fixed period of time substantially co-extensive with the application of the RF drive signal produces a fixed amount of charge per activation of the finger. Varying the duration that a control voltage is applied to the finger varies the amount of charge deposited. Similarly, varying the potential applied to the finger modulates the amount of charge delivered by the print head to the imaging member. While it is necessary for some applications such as gray scale imaging to vary the total amount of charge deposited on the imaging member, any mechanism for generating and depositing charges must be precisely controlled to provide uniform imaging and ensure a faithful reproduction free of objectionable image artifacts.
Several methods and devices have been developed to precisely control the potential and timing of the control voltage supplied to finger electrodes, discussions of which can be found in U.S. Pat. Nos. 4,841,313; 4,992,807 and 5,239,318. While existing devices and methods accurately control the potential and/or timing of the voltage provided to the fingers, inherent characteristics of the print heads may limit the effectiveness of such devices. More specifically, charge deposition print heads can exhibit a significant variation in the amount of charge generated and supplied from different charge source regions (RF electrode/finger crossings) excited by the same RF drive signal and control voltage combination.
This deviation in charge output between charge source regions requires a mechanism to individually tune each charge source region output to calibrate the print head to ensure uniform imaging. Normalizing the charge source to charge source region output requires providing a specific control voltage to each finger/electrode crossing and/or supplying the control voltage for given time intervals for each finger/electrode crossing. With existing finger driver circuits, providing different control voltages to each finger requires multiple voltage supplies, each providing a specific voltage. Given the number and density of the fingers and RF electrodes which need to be tuned, a large number of voltage sources may be required making this option relatively expensive and complex. Modifying existing drivers to vary the length of time that the control voltage is applied is a rather inexpensive and simple solution to implement. However, implementing such a solution to normalize charge output with sufficient resolution in charge output to eliminate visual artifacts in the output image comes at the expense of reduced print speed (printer throughput).
In accordance with one aspect of the present invention there is provided a circuit for driving control electrodes between a reset voltage and an adjustable extraction voltage. The circuit includes a common reset switch that is capable of being set to an xe2x80x98ONxe2x80x99 state and an xe2x80x98OFFxe2x80x99 state and that includes first and second terminals with the first terminal being connected to a high voltage source. A plurality of set switches, each being capable of assuming an on state and an off state and each including first and second terminals, are connected such that the first terminal of each set switch is connected to a corresponding one of the control electrodes and the second terminal is connected to a low voltage source. The circuit further includes a plurality of diodes, each being connected between the second terminal of the common reset switch and a corresponding control electrode. A plurality of current sources, each current source being capable of independently assuming an enabled and a non-enabled state, are connected between a high voltage source and a corresponding one of the control electrodes to supply a constant current to the control electrodes and thereby adjust the extraction voltage at the control electrode.
In accordance with another aspect of the present invention there is provided a method for driving a print head of an image forming device. The method includes (a) setting the voltage at electrodes in the print head to a nonprinting potential; (b) setting the voltage at a plurality of the electrodes to a first printing potential; and (c) charging selected ones of the plurality of the electrodes with a current source to adjust the potential at the selected electrodes to a final printing potential.
In accordance with another aspect of the present invention there is provided an imaging device including a dielectric imaging member; a print head positioned to deposit charge on the imaging member, the print head including a plurality of RF electrodes and a plurality of control electrodes; an RF driver connected to the plurality of RF electrodes, the RF driver supplying an RF voltage to the RF electrodes; and a circuit connected to the plurality of control electrodes, the circuit driving selected control electrodes between a reset voltage and an adjustable control voltage. The circuit includes a reset switch being capable of assuming a first state and a second state, the reset switch including a first terminal connected to a first voltage source; a plurality of set switches, each set switch having the capability of assuming a first state and a second state, each set switch including a first terminal connected to a second voltage source and a second terminal connected to a corresponding one of the control electrodes; a plurality of diodes, each one of the plurality of diodes connected between a second terminal of the reset switch and a corresponding one of the control electrodes; and a plurality of current sources, each current source being capable of assuming an enabled and a non-enabled state, each current source being connected to a third voltage source and a corresponding one of the control electrodes.